1. Field of the Invention
The present invention relates to an optical writing device including a photodetector for detecting the quantity of light emitted from a light-emitting point, an image forming apparatus, and a temperature calculation method.
2. Description of Related Art
In an image forming apparatus, an optical writing device is used for formation of images. One of known examples of the invention relating to conventional optical writing devices is a light-emitting device disclosed in Japanese Patent Laid-Open Publication No. 2006-201751. The light-emitting device includes a light-emitting element for light quantity detection, which is not provided for an intended use and which is exclusively used for detection of light, a light quantity detector for detecting the quantity of light from the light-emitting element for light quantity detection, and a corrector for correcting the driving conditions of a light-emitting element based on the quantity of light from the light-emitting element for light quantity detection such that the variation of the quantity of light from the light-emitting element is suppressed.
In the light-emitting device disclosed in Japanese Patent Laid-Open Publication No. 2006-201751, as the temperature increases, the quantity of light from the light-emitting element for light quantity detection also increases. As the temperature decreases, the quantity of light from the light-emitting element for light quantity detection also decreases. That is, the light-emitting element for light quantity detection has a temperature dependence. In view of such, the corrector determines an increase or decrease of the temperature of the light-emitting element for light quantity detection based on the quantity of light from the light-emitting element for light quantity detection which is detected by the light quantity detector, and corrects the driving voltage of the light-emitting element for light quantity detection. With this configuration, in the light-emitting device disclosed in Japanese Patent Laid-Open Publication No. 2006-201751, the variation of the light quantity which is attributed to the variation of the temperature of the light-emitting element can be corrected.
A light quantity detector such as a photodetector has such a characteristic that, even when the intensity of incident light is equal, the level of the output varies depending on the temperature of the light quantity detector. That is, the light quantity detector has a temperature dependence. On the other hand, in the light-emitting device disclosed in Japanese Patent Laid-Open Publication No. 2006-201751, the temperature dependence of the light-emitting element is considered, but the temperature dependence of the light quantity detector that corresponds to the light-emitting element is not considered. Therefore, in the light-emitting device disclosed in Japanese Patent Laid-Open Publication No. 2006-201751, it is difficult for the light quantity detector to accurately detect the quantity of light from the light-emitting element for light quantity detection. Without accurate detection of the quantity of light from the light-emitting element for light quantity detection, the quantity of light used for writing varies, and an image formed in the image forming apparatus deteriorates.